Hi Lee,
I think you have field curvature which is present in ED80's. The only way to fix it is a field flattener/reducer.
Check out this pics for comparison.
My ED80 before field flattener:
http://www.astrobin.com/full/88107/0/
With flattener:
http://www.astrobin.com/full/93400/C/
I bought this flattener since it it designed for the ED80. I couldn't find it locally.
https://www.teleskop-express.de/shop...ED-80-600.html